#include <bits/stdc++.h>
// 2025/01/22
// tag: 
// Author: Zhang Muen
using namespace std;

int w0, w, m;

bool check(double InterestRate){
    double p = w0;
    for (int i = 1; i <= m; i++)
        p -= w / pow(1 + InterestRate / 100, i);
    return p <= 0;
}

signed main()
{
    cin >> w0 >> w >> m;
    double l = 0, r = 300;
    while (r - l > 0.00000001){
        double mid = (l + r) / 2;
        if (check(mid))
            l = mid;
        else
            r = mid;
    }
    cout << fixed << setprecision(1) << round(l * 10) / 10 << endl;
    return 0;
}